Myeloperoxidase (MPO) exerts an antimicrobial effect when combined with H2O2 and an appropriate oxidizable substance such as a halide, and this antimicrobial system appears to play an important role in neutrophilic polymorphonuclear leukocytes (PMNs). The superoxide anion and hydroxyl radical, as well as a number of oxygen-independent antimicrobial systems, also contribute to the microbicidal activity of the cell. We propose to study the properties, mechanism of action and biological significance of the oxygen-dependent antimicrobial systems with particular regard to the role of singlet molecular oxygen. Two general methods, the identification of singlet oxygen-dependent chemical conversions and the spectral analysis of the emitted light, will be employed for the detection of singlet oxygen formation by the MPO system, by superoxide anion-dependent reactions, by intact PMNs and by subcellular fractions of the PMN. The conversion of methional to ethylene will be employed in a study of hydroxyl radical formation by MPO-deficient leukocytes. These and other procedures will be used in an evaluation of neutrophil function in patients with an increased susceptibility to infection. The long term goals are: 1) to determine the role of these antimicrobial systems in the killing of microorganisms by neutrophils; 2) to determine whether alterations in the components of these systems might contribute to the pathogenesis of disease; and 3) to determine whether manipulations of these systems might influence the course of disease.